Solar EUV Radiation Research Articles

Dissipation of the rare gases contained in the primordial Earth's atmosphere

If the Earth was formed by accumulation of rocky bodies in the presence of the gases of the primordial solar nebula, the Earth at this formation stage was surrounded by a massive primordial atmosphere (of about 1 × 10 26 g) composed mainly of H 2 and He. We suppose that the H 2 and He escaped from the Earth, owing to the effects of strong solar wind and EUV radiation, in stages after the solar nebula itself dissipated into the outer space. The primordial atmosphere also contained the rare gases Ne, Ar, Kr and Xe whose amounts were much greater than those contained in the present Earth's atmosphere. Thus, we have studied in this paper the dissipation of these rare gases due to the drag effect of outflowing hydrogen molecules. By means of the two-component gas kinetic theory and under the assumption of spherically symmetric flow, we have found that the outflow velocity of each rare gas relative to that of hydrogen is expressed in terms of only two parameters — the rate of hydrogen mass flow across the spherical surface under consideration and the temperature at this surface. According to this result, the rare gases were dissipated below the levels of their contents in the present atmosphere, when the mass loss rate of hydrogen was much greater than 1 × 10 17 g/yr throughout the stages where the atmospheric mass decreased from 1 × 10 26 g to 4 × 10 19 g.

Calculated ionospheric variations due to changes in the solar EUV flux measured by the AEROS spacecraft

The solar EUV radiation from the whole solar disk for wavelengths between 15.5 and 103.76 nm was measured by the AEROS-EUV spectrometer during the period December 1972 through August 1973. These measurements show a long-term decrease in solar EUV output with decreasing solar activity, a 27-day variation, and day-to-day and other short-term fluctuations. These solar EUV flux data are used with a model of the ionospheric E- and F-regions to calculate the variation of ionospheric properties that are associated with the changes in the measured solar EUV flux. The model calculates the variations in electron and ion density, electron and ion temperature, photoelectron spectra, various airglow emission rates, and the minor neutral constituents N( 2D) , N( 4S) and NO. The results show that in general the magnitude of all ionospheric and atmospheric properties increases with increasing solar EUV flux. However, percentage changes of the calculated properties are different from each other and also from the percentage change of the measured total solar EUV energy flux. This occurs because a nonuniform variation of the solar flux with wavelength is observed over the EUV spectrum by the AEROS-EUV spectrometer and altitude-dependent aeronomic processes are sensitive to the detailed flux values within the solar EUV spectrum.

The AFROS EUV Measurements

Aboard AEROS-A and B the solar EUV radiation was monitored in the wavelength range from 106-16nm with a planar grating spectrometer. During the measuring periods three types of solar flux variations were recorded: Long-term variations with solar cycle activity, medium-term variations with solar rotation and short-term variations with solar events. These variations differ from those of the classical solar indices. The determination of the absolute values of the solar flux is based on the calibration by means of synchrotron radiation and an inflight calibration of the multipliers. Other instrumental modes of operation performed EUV extinction measurements to derive densities of atomic oxygen.

The Martian ionosphere in light of the Viking observations

A theoretical model has been constructed in which the ion density and the ion and electron temperature distributions are calculated by solving the coupled continuity‐momentum equations and the coupled energy equations. The latest experimental results from the Viking 1 and 2 landers are used to vary some of the parameters in the model in order to obtain agreement between the theoretical and experimental results. It is found that solar EUV radiation alone is not able to maintain the observed high ion temperatures. It was also established that the energy coupling between the electron and ion gas is insufficient to account for the measured ion temperatures even in the presence of very large electron temperatures. Direct heat input to the ion gas, probably due to solar wind‐ionosphere interactions, can result in ion temperature values in reasonable agreement with the observations. The ion densities calculated with the present model agree well with the Viking observations in the chemically controlled region, but at higher altitudes, dynamic transport processes need to be invoked to achieve consistency among the observed and calculated temperature and density values.

Meteor radar rates, geomagnetic activity and solar wind sector structure

THERE is evidence which indicates1–4 that the occurrence rate of meteor radar echoes varies inversely with solar cycle activity, this phenomenon can be explained5–6 by a long-term variation in the atmospheric density gradient at the meteor ablation level. Inferred changes in the atmospheric properties are attributed to a long-term variation in the solar EUV radiation. Although the long-term, solar cycle changes in radar-determined meteor rates show an inverse correlation with sunspot numbers and other measures of solar activity, such correspondence is not readily evident in the day-to-day meteor radar data. One reason for this is the inability of most solar activity indices to measure the relevant solar parameters involved in short-term solar–terrestrial effects. The study of solar-influenced meteor rate changes on a time scale of days has, therefore, not received adequate attention. Evidence for a short-term dependence of meteor radar rates on geomagnetic activity and solar corpuscular radiation is presented here.

Atmospheric heating by solar EUV radiation

A diurnal model of the mid-latitude ionospheric R region is used to calculate the diurnal variation of the neutral gas heating rates and neutral gas heating efficiency for conditions similar to those over Millstone Hill on March 23-24, 1970. The calculations show that the absorbed solar EUV (wavelength less than or equal to 1025 A) energy is almost equally split between photoelectrons and ion pair production. Photoelectrons heat the ambient electron gas by Coulomb collisions and by the quenching of certain excited ion species, whereas the ion gas is primarily heated by collisions with hot electrons and by chemical reactions. Heating processes above 300 km, between 170 and 300 km, and below 170 km are identified.

The effective recombination coefficient measured in the auroral E-region during a sudden commencement electron precipitation event

Incoherent scatter radar observations are utilized to derive the effective recombination coefficient and the ion production rate in the auroral E-region during a sudden commencement precipitation event. The recombination coefficient is found to be (1.1 ± 0.3) × 10 −7 cm 3 sec −1 in good agreement with present laboratory measurements of the dissociative recombination coefficient of O 2+ for an electron temperature close to 500 K. The ion production rate associated with the SC is found to be at least 4.5 × 10 4 el cm −3 sec −1 at 110 km, this corresponds to an enhancement above the background ionization by more than a factor of 10. The background ionization is also larger than expected from solar EUV radiation due to ionization associated with a PCA event present.

An estimate of the global‐scale joule heating rates in the thermosphere due to time mean currents

An estimate of the global‐scale joule heating rates in the thermosphere is made based on derived global equivalent overhead electric current systems in the dynamo region during geomagnetically quiet and disturbed periods. The total neutral gas heating rate due to joule dissipation is proportional to the Pedersen conductivity and the square of the total electric field perpendicular to the geomagnetic field line. The equivalent total electric field distribution is calculated from Ohm's law as applied to the dynamo region using the equivalent overhead currents estimated from geomagnetic data and the components of the two‐dimensional conductivity tensor determined from global empirical models of the electron and neutral gas number density distributions. The global‐scale joule heating rates are calculated for various monthly average periods in 1965. The calculated joule heating rates maximize at high latitudes in the early evening and postmidnight sectors. During geomagnetically quiet times the daytime joule heating rates are considerably lower than heating by solar EUV radiation. However, during geomagnetically disturbed periods the estimated joule heating rates increase by an order of magnitude and can locally exceed the solar EUV heating rates. The results show that joule heating is an important and at times the dominant energy source at high latitudes. However, the global mean joule heating rates calculated near solar minimum are generally small compared to the global mean solar EUV heating rates.

Dynamics of the F-region

A discussion on the dynamics of the F-region is given from viewpoints of thermospheric motions, plasma transport, their effects on the F-region variations, and electrodynamics. Dynamics of the F-region are characterized by major heat sources arising from solar EUV radiations and solar winds, and also by strong coupling between neutral and ionized constituents. Dynamical effects on diurnal, seasonal and storm variations of the F-region are briefly discussed. Dynamo action in the F-region and resulting polarization electric field, electric current, thermospheric wind, plasma transport and plasma energetics are discussed.

Incoherent scatter measurements ofEregion conductivities and currents in the auroral zone

Data taken by incoherent scatter radar have been used to investigate ionospheric conductivities and electrical currents. During quiet days, the conductivities appear to vary in a way consistent with ionization arising from solar EUV radiation. In the evening hours, enhancements in the northward electric field are found to precede small increases in the conductivities. Strong enhancements of the Hall conductivity relative to the Pedersen conductivity occur during negative bays when the electric field is in a southwestward direction. The ionospheric currents calculated in the geomagnetic east-west direction are in good agreement with the H component measured by a nearby magnetometer; this result indicates that the current causing the ground level magnetic fluctuations is a broad horizontal sheet current. The north-south ionospheric current, however, consistently disagrees with the observed D component in a manner that cannot easily be explained unless currents parallel to the earth's magnetic field are present.

Convection electric fields and polar thermospheric winds

Use of the qualitative ideas of convection electric fields over the earth's polar regions to demonstrate the importance of ion drag in establishing a thermospheric wind system. Recent measurements indicate that uniform electric fields of 10 to 40 mV/m are a regular feature of the polar-cap ionosphere. Calculations of the neutral thermospheric wind, using these measured fields in a simple ionospheric model, have been made. The time scale for motion of the neutral gas ranges from less than 1 hour at F-region heights to about 2 hours in the dynamo region of the ionosphere. It has been found that the viscosity of the atmosphere is important in determining the winds in the dynamo region. Results are given that show ion-temperature enhancements of hundreds of degrees that are due to ion-neutral frictional effects. In addition, the total deposition rate of convection energy in the polar thermosphere is shown to be of the same order of magnitude as that due to absorption of solar EUV radiation. The implications of these results for the dynamics and energetics of the thermosphere are discussed.

Interstellar matter and the location of the shock front

The initially supersonic flow of the solar wind passes through a magnetic shock front where its velocity is supposed to be reduced to subsonic values. The location of this shock front is primarily determined by the energy density of the external interstellar magnetic field and the momentum density of the solar wind plasma. Interstellar hydrogen penetrating into the heliosphere undergoes charge exchange processes with the solar wind protons and ionization processes by the solar EUV radiation. This results in an extraction of momentum from the solar wind plasma. Changes of the geometry and the location of the shock front due to this interaction are studied in detail and it is shown that the distance of the magnetic shock front from the Sun decreases from 200 to 80 AU for an increase of the interstellar hydrogen density from 0.1 to 1.0 cm −3. The geometry of the shock front is essentially spherical with a pronounced embayment in the direction opposite to the approach of interstellar matter which depends very much on the temperature of the interstellar gas. Due to the energy loss by the interaction with neutral matter the solar wind plasma reduces its velocity with increasing distance from the Sun. This modifies Parker's solution of a constant solar wind velocity.

The ionosphere during a solar eclipse

Two different partial solar eclipses are examined, each for two different stations. A comprehensive version of the steady state ionospheric continuity equation is solved for theoretical comparison. It is shown that it is not possible to reproduce the ionospheric eclipse effects satisfactorily using solar EUV radiation as the only source of ionization. The resultant drop in N m F2 is so much larger than is observed that another source, unaffected by the eclipse, must be acting throughout the eclipse. This is found to apply to both mid- and high-latitude stations. The inclusion of a corpuscular source of ionization yields theoretical results in better agreement with the observed than when it is not included. The second source is the major one in the F2-region and contributes negligibly to the production rate below 165 km.

Observations and computations of twilight helium 10,830-Angstrom emission

Observations of helium 10,830-A radiation with a grille spectrometer at Socorro, New Mexico, from 1967 to 1970 are presented. The data are discussed in light of new computations of the excitation and emission profiles. The observational data show an intensity increase of a factor of 3 during this period that is closely related to a corresponding change in the solar EUV radiation (λ<380 A). A large seasonal variation is discussed that is caused in part by a 5∶1 variation in the neutral helium abundance and that is augmented by seasonal variation in the conjugate photoelectron flux. In addition, a diurnal variation of the helium abundance of about 30% with a maximum in the morning is suggested.

Response of winds in the 90- to 140-km altitude region to variations in solar activity

Winds determined from 52 photographically tracked trails of rocket-borne chemical releases in the 90- to 140-km altitude range, launched between October of 1962 and June of 1966, are used in both a correlation analysis and a linear regression analysis with several solar and geomagnetic disturbance indices. Results indicate that the altitude interval considered may be broken up into two distinctly different regions of solar influence: the regions above and below 110 km. Significant increases in the geomagnetic index Kp over a 15-hour interval centered on a time lag of 21 hours after increases in atmospheric circulation below 110 km suggest either a dynamo type of interaction or an increase in the geomagnetic field strength, produced by corpuscular radiation which lags behind the electromagnetic radiation and accompanying atmospheric circulation variations. Apparently continuous associations, with indicated periods of about one solar rotation, between both the solar radio noise index F10.7 and the Zurich sunspot number Rz and the northward wind component magnitude above 110 km indicate that long-term variations in the winds in this region are related to similar variations in solar EUV radiation.

The magnetospheres of Jupiter and Earth

The magnetic fields of Earth and Jupiter carve cavities out of the solar wind, which in turn influences the dynamics and structure within the magnetospheres of these planets. From a knowledge of the factors which influence the terrestrial magnetosphere, expectations for the Jovian magnetosphere are deduced. The shape of the Jovian magnetosphere should be similar to that of Earth, with the size of the magnetosphere slightly larger relative to the size of the plan and the size of the auroral zone slightly smaller. The fraction of the incident solar wind energy injected into the magnetosphere should be comparable for Earth and Jupiter. Major dissimilarities may occur because of the following fundamental differences: 1. (1) The solar-wind-induced plasma flow should be smaller than the corotational flow essentially throughout the Jovian magnetosphere. 2. (2) The centrifugal and gravitational forces balance at a point well within the region of Jupiter's magnetosphere dominated by corotation. 3. (3) Drift periods for energetic particles trapped in the magnetosphere are generally smaller than the rotation period for Earth, but much much larger for Jupiter. 4. (4) In the polar-cap regions of Earth, energy deposition into the ionosphere from the solar wind and solar EUV radiation are comparable, but for Jupiter's polar cap ionosphere, the solar wind input shoul be clearly dominant.

A theory of thermospheric dynamics—II: Geomagnetic activity effect, 27-day variation and semiannual variation

The geomagnetic activity effect, the 27-day variation and the semiannual variation of the thermospheric density are nearly independent of local time. Therefore, they are treated by a one dimensional model in order to find out the height distribution and the amount of energy necessary to create the observed amplitudes and phases of the neutral density between 120 and 400 km altitude. Three types of heat energy sources have been considered which simulate energy input: I at the bottom of the thermosphere; II within the thermosphere; and III at the top of the thermosphere. The geomagnetic activity effect and the 27-day variation can be described as generated by heat source II which is interpreted as resulting from corpuscular radiation into the aurora zones and from solar active region EUV radiation, respectively. The semiannual effect can be described by heat source I and is interpreted as resulting from dissipated energy of a tidal wave from the lower atmosphere. The leakage of the energy of this tidal wave into the thermosphere shows a semiannual variation which creates the observed density variations. Quantitative calculations give the amount of energy of the various heat sources, amplitudes and phases of the generated density and temperature variations and their dependence on solar activity.

The electrostatic charge of interplanetary grains

The equilibrium potential of an interplanetary grain is governed essentially by the balance between the photoelectric ejection of grain electrons by solar quanta and the capture of solar-wind electrons by the grain. When the photoelectric effect of the actual EUV and X-ray radiation of the Sun are accounted for, the equilibrium potential of a metallic grain during years of a quiet Sun is found to range from about +7 to +18 V, depending on whether the instantaneous flux of the solar wind is near its maximum or its minimum. A working typical value is +12 V, larger by a factor of two or three than other recent estimates. It is estimated that during the peak of the solar cycle the enhanced EUV solar radiation may result in grain potentials up to +30 Von occasions when the solar-wind flux is low. As is well known, one of the consequences of the finite electrostatic charge is that the solar wind exerts a Coulombic drag force on an orbiting interplanetary grain, which adds to other perturbative forces to reduce by a factor of about 1.5 the time taken for a grain to spiral inward toward the Sun until it is lost to the zodiacal cloud.

The delay between solar activity and density changes in the upper atmosphere

Since 1958 it is known that there exists a response time of the upper atmosphere to changes in solar activity. This response time is best described as the lag between the 27-day variation of solar decimeter flux and the observed density changes of the upper atmosphere. Roemer obtained as a mean observational value for this lag 1.0 ± 0.12 days. Volland's simplified version of the Harris-Priester model of the upper atmosphere is used to calculate the delay which can be expected from theory. Only the effect of solar EUV radiation is taken into account. A possible influence of the corpuscular component of the solar radiation is not included in our estimate. The calculations are carried out for the Harris-Priester model with solar activity index F = 200 and a variation of ΔF F = O.1 . The resulting delay is 0.6 days. The calculated amplitude of the variations of the diurnal average temperatures during the solar 27-days cycle is in very good agreement with Jacchia's empirical formula.

The response of the upper atmospheric temperature to changes in solar EUV radiation and geomagnetic activity

The upper atmospheric neutral temperature derived from satellite drag is analyzed with a view to determine the relative importance of solar EUV and magnetic activity on heating of the upper atmosphere. As the indices of solar EUV radiation, the radio fluxes obtained from the ground based observations in the frequency range of 200–9400 Mc/s are used. Also for the period March 7–May 15, 1962, a direct comparison is made with the EUV flux obtained from the OSO 1 Satellite. As the index of magnetic activity the planetary magnetic index K p , is used. The data have been analyzed by separating the long period components of 27 days and above from the short term components by using a 5 day running mean filter. A cross correlation study between the various parameters shows that the long term variations in temperature are strongly correlated with the similar variations in the radio flux in the frequency range of 1000–3750 Mc/s, the correlation being maximum at 1000 Mc/s. The short term variations in temperature, on the other hand are strongly correlated with those in ∑ K p , the daily sum of K p indices and not with the radio flux at any of the frequencies considered. The correlation between temperature and EUV flux in the short term components is somewhat better though not as striking as the long term components. An empirical relation between temperature and the changes in radio fluxes and ∑ K p based on the above correlation study shows that T N = 524 + 2.73 S 10.7 + 1.77 (∑K p − ∑K p ) or T N = 470 + 6.53 S 30 + 1.77 (∑K p − ∑K p ) where T N is the nighttime minimum temperature; S 10.7, S 30 and ∑K p are the 5-day running means of 2800 Mc/s radio flux, 1000 Mc/s radio flux and ∑ K p respectively. The present investigation leads to the conclusion that short term variations in temperature are closely associated with the changes in magnetic activity represented by K p , and all the systematic variations temperature including the so called semi-annual variation can be fully accounted for in terms of the similar variations in solar flux.